In order to improve fuel economy, a fuel cut-off control for stopping fuel supply to an engine during running, and a coasting control for coasting the vehicle by disconnecting the engine from a drive line during running have been in recent years. Specifically, the fuel cut-off is carried out to stop fuel supply to the engine given that an accelerator is returned to close an accelerator valve completely, and that an engine speed is higher than an idling speed so that a vehicle speed is higher than a predetermined speed. Under the fuel cut-off control, although the combustion of fuel in the engine is stopped, the engine is still rotating by a torque from the drive wheels. Therefore, a braking torque resulting from a pumping loss of the engine and a friction torque is applied to the drive wheels. Consequently, an engine braking force is applied to the vehicle.
For example, the coasting control is carried out when the accelerator is returned during running to coast the vehicle without rotating the engine passively by bringing the clutch into disengagement disposed between the engine and the drive wheels. Therefore, under the coasting control, the engine braking force will not be applied to the vehicle so that the vehicle is allowed to coast utilizing an inertia energy.
The coasting control may be carried out not only by stopping the fuel supply to the engine but also without stopping the fuel supply to the engine while lowering the engine speed close to the idling speed. In case of carrying out the coasting control while stopping the engine, the fuel will not be consumed during coasting the vehicle so that the fuel economy can be improved significantly. By contrast, in case of carrying out the coasting control without stopping the engine, the fuel economy will not be improved as the case of stopping the engine. In this case, however, it is not necessary to arrange additional devices for insuring a required oil pressure under cessation of the engine such as an electric oil pump, a hydraulic accumulator etc. Therefore, the coasting control can be carried out easily without requiring additional modification of the vehicle and additional equipment. In the following explanation, the coasting control without stopping the engine will be called especially the “neutral coasting control” or the “N coasting control”.
For example, Japanese Patent Laid-Open No. 2011-163535 describes a controller for mechanical automatic transmission configured to carry out the coasting control. According to the teachings of Japanese Patent Laid-Open No. 2011-163535, an input shaft of a geared transmission is connected to an output shaft of an engine though a friction clutch, and an alteration of engagement state of the clutch and a speed change operation of the transmission are carried out automatically. To this end, a coasting of the vehicle is determined based on a vehicle speed, an execution of a speed change of the transmission, and an accelerator opening. Given that the coasting of the vehicle is continued for a predetermined period of time, the clutch is brought into disengagement and the vehicle speed is lowered to an idling speed. In case the vehicle is not coasting after disengaging the clutch, a target gear stage of the transmission is established according to the vehicle speed and the accelerator opening, and then the clutch is brought into engagement.
Japanese Patent Laid-Open No. 2005-140076 discloses a control device for a vehicle mounting an automatic transmission comprising a torque convertor having a lockup clutch. In case a driving condition of the vehicle meets a predetermined requirement, fuel cut-off is carried out to stop supplying the fuel to the engine. When the lockup clutch is in an engaging state or a slipping state, the fuel supply is stopped earlier than when the lockup clutch is in the engaging state. In addition, according to Japanese Patent Laid-Open No. 2005-140076, after a predetermined delay time since the lockup clutch has been released, fuel supply to a plurality of cylinders of the engine is stopped serially.
For example, Japanese Patent Laid-Open No. 2010-24773 describes a control for the neutral coasting. The coasting controller taught by Japanese Patent Laid-Open No. 2010-24773 is configured to bring the clutch disposed between the engine and the driving wheels into disengagement when the engine is not involved in propelling the vehicle, and to allow the vehicle by idling the engine. In addition, the coasting controller is further configured to terminate the coasting control regardless of a satisfaction of the terminating condition if a difference between the vehicle speed at the commencement of the coasting control and the current vehicle speed exceeds a predetermined threshold value during execution of the coasting control.
Japanese Patent Laid-Open No. 2011-106378 describes a vehicular controller configured to carry out a cylinder deactivation of an engine comprising a plurality of cylinders. The vehicular controller taught by Japanese Patent Laid-Open No. 2011-106378 is configured to execute a deceleration control by altering a speed ratio and number of deactivated cylinders based on the required deceleration during stopping fuel supply to the engine under the idle off state. Japanese Patent Laid-Open No. 2011-106378 further describes a fact that a pumping loss is reduced with an increase in the deactivated cylinders and that the deceleration is changed in a mild manner during the deceleration control.
As described, according to the teachings of Japanese Patent Laid-Open No. 2011-163535, the friction clutch is brought into disengagement during the coasting of vehicle so that the vehicle is allowed to be propelled by inertia force, and the engine speed is lowered to the idling speed. Consequently, fuel economy of the vehicle is improved. During the coasting control, however, a power transmission between the engine and the drive wheels is interrupted. In this case, if the vehicle is running down a downslope, an engine brake will not be applied to the vehicle and the vehicle speed is continuously increased. That is, the driver may not feel deceleration of the vehicle. Thus, according to the conventional coasting control taught e.g., by Japanese Patent Laid-Open No. 2011-163535, the driver may feel uncomfortable feeling or fear if the vehicle enter into a downgrade and the vehicle speed is therefore increased.